1. Field of the Invention
The present invention relates generally to a process for fabricating membrane filters and a membrane filter, and more particularly to a process for fabricating a porous membrane using a nanostrand sheet and a membrane filter comprising that porous membrane.
2. Description of the Prior Art
Membrane filters are frequently used in a variety of applications where there are the needs for filtration and separation such as air cleaning, and solid-liquid separation.
In particular, membrane filters capable of separating nanometer-scaled fine particles and organic molecules find wide applications to the production of drinking water and industrial water, sewage purification, the manufacturing of foodstuffs and chemicals, medical purification membranes, etc.
Ultrathin membrane filters are now expected to achieve fast permeation of liquids, and there is a great likelihood that the treatment time taken for separation will be much curtailed. They also lead to much savings of the energy necessary for separation, and there is every reason to expect that they will provide separation membranes in association with the next-generation energy creation.
So far, several methods have been known for the fabrication of membrane filters.
Reverse osmosis membranes capable of removing organic molecules and dissolved ions are generally obtainable by making use of the polycondensation of monomers at a liquid/liquid interface. This method provides a large-area polymer membrane having a thickness of a few tens of nanometers, which is widely used as a water treating membrane.
However, pores inside the polymer membrane are as small as the rate of permeation of liquids is too slow, resulting in the need for high pressure for the permeation of liquids.
With the phase transition of a polymer solution, there are membrane filters obtained that are capable of separating fine particles or organic molecules of up to 200 nm in diameter. These filters, called ultrafilter membranes, are used in a wide range of applications.
However, the ultrafilter membranes have generally a thickness of greater than a few tens of micrometers; hence, the rate of filtration is very slow.
In recent years, for this reason, studies of membrane filters having fast filtration and high energy efficiency have been made extensively and intensively.
Referring typically to the “alternate adsorption method” wherein positively and negatively charged polymer electrolytes are alternately adsorbed onto a solid substrate, there is a polymer membrane obtained with its thickness controlled on a nanometer scale, and its application to membrane filters is now under study.
Spin coating makes it possible to form polymer membranes of the order of 10 nm, which are not only used as resists for semiconductor fabrication but are also studied as to whether or not they may be used as ultra-thin separating membranes.
However, those methods have difficulty in continuous fabrication of nanometer-scaled, large-area membranes; they are still not established as an industrial fabrication process for membrane filters.
For the fabrication of nanofibrous membrane filters, electrospinning now attracts attention.
With this technique, there are membrane filters obtained that comprise nanofibers of a few tens of nanometers in width, but pores formed in inter-nanofiber gaps are larger than the width of each nanofiber, rendering it difficult to fabricate a membrane filter having a pore diameter of up to 10 nm.
A recent report by Wu et al. shows that a dispersion of single wall carbon nanotubes in water is filtrated so that a nanofibrous free-standing film can be produced (Non-Patent Publication 1).
The nanotube used by them is partially oxidized on its surface so that it can be dispersed in water using a surface active agent. As the dispersion is filtrated through a cellulosic filter, then fully washed, and then immersed in acetone for dissolution of the filter, it yields a transparent, electrically conductive nanofibrous free-standing film of 80 nm in thickness and 10 cm in diameter.
The method proposed by Wu et al. is regarded as a fabrication process for membrane filters by relying upon the “filtration” of a nanofibrous substance dispersion. This method provides a simple, very efficient process for fabricating large-area membrane filters, as is the case with “papermaking”, and would enable the thickness of membrane filters to be controlled by the concentration and amount of the dispersion used as well.
For the fabrication of membrane filters by the “filtration”, however, the nanofibrous substance must have a high aspect ratio.
For the fabrication of an ultrathin membrane filter having a pore diameter of up to 10 nm, ultrafine nanofibers are inevitable, and there is the need for those nanofibers to be dispersed uniformly in a solvent.
In addition, the nanofibers must be rigid and structurally stable against suction filtration.
Never until is such a substance as satisfying these conditions found except single wall carbon nanotubes.
For the nanofibers capable of being used with the filtration method, the inventors have focused on independently developed metal hydroxide nanostrands.
The nanostrand has a diameter of up to 2.5 nm, and has strongly positive charges on its surface so that it is present while dispersed in water. For this reason, it can be formed by filtration into a sheet configuration as is the case with an ultrafine carbon nanotube.
Still, the nanostrand sheet, because of being formed of a metal hydroxide, has a demerit of being easily dissolved in an acidic aqueous solution; so it has been expected to be hardly applied to membrane filters.
To solve the aforesaid problem, we have coated an electrically conductive polymer around individual nanostrands in water to prepare a composite fiber improved in terms of chemical stability. We have also prepared an ultrafilter membrane having a thickness of a few tens of nanometers by filtration of such composite fibers (Non-Patent Publication 2).
Moreover, we have revealed that nanostrands and a water-soluble substance (proteins or anionic nanoparticles, dyes, fullerene, nanotubes (hereinafter called the proteins or the like”) are mixed together in water to prepare a composite fiber so that various free-standing films or membranes can be fabricated by the filtration of that composite fiber (Non-Patent Publication 3).
In particular, it has been shown that the free-standing membrane formed by the filtration of the composite fibers of nanostrands and protein provides an effective ultrafilter membrane for fast filtration of organic molecules (Non-Patent Publication 4), and it has also been confirmed that as the free-standing membrane is used to make a deposition of filter cakes of a polymer such as a dendrimer and the filter cakes are crosslinked with a difunctional crosslinking, it can yield free-standing membrane of a thin double-layer structure (Patent Publication 1).
Thus, the composite fiber prepared from nanostrands and the proteins or the like yields an ultrathin free-standing membrane by means of filtration.
For this method, however, there was the need for designing interactions between the nanostrands and the protein or the like.
For instance, the nanostrand has a lot of positive charges on its surface; so it could form a composite fiber with a dye molecule having multiple negative charges, but it had difficulty forming a composite fiber with a dye molecule having a neutral or positive charge.
When the electrically conductive polymer was coated around the nanostrand, too, there was the need for addition of an additive such as polystyrene sulfonate so as to bring about electrostatic interactions with the nanostrand. In addition, the electrically conductive polymer to be coated was limited to polypyrrole or polyaniline alone.
Moreover, there were a lot of charges on the surface of the composite fiber coated with the electrically conductive polymer and the proteins or the like, because it had to have been dispersible in water, ending up with a problem of easily absorbing a substance having opposite charges.